AVR-Chat

[1]
While perusing the ATmega168A documentation, I discovered that NOWHERE 
is it mentioned
concerning the byte order when a push or pop, call or return instruction 
is issued.

So the first question is simple:  When I call a subroutine, which byte 
of the programme
counter is pushed first:  The high byte or the low byte?  The important 
reason is below.


[2]
The following question is a little more complex:

I sometimes write subroutines (on other CPUs) that take either constant 
arguments
or pointers to variables coded inline with the subroutine call.

An example of such a call is shown below:

     CALL some_procedure
     .dw    argument_1       ; constant or pointer argument
     .db    "some_text",0    ; constant text argument
     ....
return_point:

What happens is this:

The saved programme counter is popped into an index register.  Bytes or 
words are
read using the index and the index is incremented accordingly.
When the procedure is finished, the index is already pointing to the 
correct return
point, so doing an indirect jump to the contents of the index register 
returns you
to the caller.  Alternatively, I can push the index back onto the stack 
and do a
normal return instruction.

I'm thinking that such code would look like this:

     rcall  some_procedure
     .dw    argument_1
     .dw    argument_2
     ...
     ...

some_procedure:
     pop    ZL           ; I'm not sure if this is the right order.
     pop    ZH           ; when the stack is popped, it is back to the 
state it
;                       ; was in before the call.
     lsl    ZL           ; times two because LPM only extracts bytes.
     rol    ZH           ; we lose the high bit so only the lower 32K
                         ; can be addressed
     ...
     procedure code goes here, using LPM reg,Z+ to load the registers
     ...
     adiw   Z,1          ; bump return address past last byte of parameters
     lsr    ZH
     ror    ZL           ; adjust back to word format
     ijmp                ; return to caller

The question is this:  Is this code correct?

Mostly, I use this with output routines that are called on to display 
large amounts
of fixed text, such as instructions or help screens.

[3]
I have one final question that has nothing to do with stacks:  When 
using AVRstudio,
what is the correct programming protocol for use with an Arduino Mega 
2560 board?

fireweaver




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